Recently, as a storage apparatus of a computer, a storage apparatus in which high speed writing can be performed, and which has no limits of the number of times of writing, and further which is nonvolatile has been desired. As a storage apparatus having these performances, a magnetic storage apparatus using a ferromagnetic tunnel junction device formed by laminating a fixed magnetized layer and a free magnetized layer with a tunnel barrier layer put between them has attracted attention.
Such a magnetic storage apparatus using a ferromagnetic tunnel junction device is a storage apparatus using a giant magnetoresistive effect caused by the ferromagnetic tunnel junction device. The resistance values of the tunnel barrier layers in the following cases differ from each other: the case where the free magnetized layer of the ferromagnetic tunnel junction device is magnetized into the same direction as the magnetizing direction of the fixed magnetized layer, and the caser where the free magnetized layer is magnetized in the opposite direction to the magnetizing direction of the fixed magnetized layer. By utilizing the difference of the resistance value, two different states of the magnetizing directions are formed in the following two cases: a case where the free magnetized layer is magnetized into the same direction as the magnetizing direction of the fixed magnetized layer and the other case where the free magnetized layer is magnetized into the opposite direction to the magnetizing direction of the fixed magnetized layer. The magnetic storage apparatus stores data by relating such two different states of the magnetizing directions to two storage data of “0” and “1”, respectively.
More specifically, the magnetic storage apparatus using the ferromagnetic tunnel junction device disposes ferromagnetic tunnel junction devices on a semiconductor substrate with predetermined spaces at front and back positions and at right and left positions. The magnetic storage apparatus further wires a bit line elongating toward a direction orthogonal to the magnetizing direction of the fixed magnetized layer at the upper portions of each of the ferromagnetic tunnel junction devices. The magnetic storage apparatus also wires a writing word line and a reading word line, both elongating toward the magnetizing direction of the fixed magnetized layer at the lower portions of each of the ferromagnetic tunnel junction devices. Moreover, the magnetic storage apparatus connects a writing current generation circuit to each of the bit lines and each of the writing word lines. On the other hand, the magnetic storage apparatus connects a reading resistance detection circuit between each of the bit lines and each of the reading word lines.
Then, a current generated by the writing current generation circuit is applied to a bit line and a writing word line severally, and thereby generating bit line magnetic force orthogonal to the bit line and word line magnetic force orthogonal to the writing word line. Writing magnetic force generated by combining the bit line magnetic force and the word line magnetic force is applied to the free magnetized layer of the ferromagnetic tunnel junction device to magnetize the free magnetized layer into the same or opposite direction as or to the magnetizing direction of the fixed magnetized layer. Thereby, the writing of storage data into a ferromagnetic tunnel junction device is performed.
On the other hand, the resistance value of a tunnel barrier layer is detected by the reading resistance detection circuit on the basis of the magnitude of the current which flows through the ferromagnetic tunnel junction device when a predetermined voltage is applied between the bit line and the reading word line. Then, the magnetizing direction of the free magnetized layer is determined on the basis of such resistance value. Thus, the reading of the storage data in the ferromagnetic tunnel junction device is performed.
When storage data is written in the ferromagnetic tunnel junction device in the magnetic storage apparatus having the configuration described above, it is impossible to magnetize a free magnetized layer without applying the writing magnetic force having a predetermined magnitude to the free magnetized layer. Consequently, it has been impossible to write any storage data into the ferromagnetic tunnel junction device without the writing magnetic force.
This is described by means of a storage state explanatory view shown in FIG. 9. The storage state explanatory view is a diagram showing, by means of an abscissa axis expressing the magnitude of the word line magnetic force and an ordinate axis expressing the magnitude of the bit line magnetic force, either value of “0” and “1” storage data is taken according to the direction of writing magnetic force as the combination of the word line magnetic force and the bit line magnetic force. When the writing magnetic force is oriented toward the direction above the abscissa axis, it is indicated that storage data “0” is stored. When the writing magnetic force is oriented toward the direction below the abscissa axis, it is indicated that storage data “1” is stored.
When the writing magnetic force exists in a region enclosed by an asteroid curve around the origin as the center thereof (hereinafter, the region will be referred to as “unstorable region”) in the storage state explanatory view, the magnitude of the writing magnetic force is too small to magnetize the free magnetized layer, and it has been impossible to write storage data into the ferromagnetic tunnel junction device.
Accordingly, a conventional technique has set the magnitude of the writing magnetic force in order that the writing magnetic force may be situated on the outside of the unstorable region enclosed by the asteroid curve, and has applied the current necessary for generating the writing magnetic force to a bit line and a writing word line severally.
Moreover, the conventional technique has fixed the magnitude of currents flowing in the bit line and in the writing word line regardless of the value “0” or “1” of the storage data to be written for simplifying the writing current generation circuit, and the writing of the storage data has been performed only by inverting the flowing direction of the current through the bit line.
However, there is the case where the unstorable region of the ferromagnetic tunnel junction device shifts upward or downward, or rightward or leftward, on the storage state explanatory view owing to causes in manufacturing processes of the ferromagnetic tunnel junction device. There has been the possibility of the impossibleness of writing storage data into the ferromagnetic tunnel junction device because the magnitude of the writing magnetic force being within the unstorable region actually despite being designed to be situated on the outside of the unstorable region.
Then, a conventional magnetic storage apparatus using the ferromagnetic tunnel junction device has made it possible to write storage data into the ferromagnetic tunnel junction device by adding a margin of a certain degree to the magnitude of the writing magnetic force to make the writing magnetic force act on the free magnetized layer even if the unstorable region of the ferromagnetic tunnel junction device has somewhat shifted. The conventional magnetic storage apparatus generates the writing magnetic force with margins caused by the application of a current having an unchanging fixed magnitude to a bit line and a writing word line.
That is, because the conventional magnetic storage apparatus using the ferromagnetic tunnel junction device has been configured to be unable to change the magnitude of the writing magnetic force, the conventional magnetic storage apparatus has generated the writing magnetic force which is too sufficient to magnetize the free magnetized layer by making the current larger than being necessary to flow through the bit line or the writing word line. Thereby, the current flowing through the bit line or the word line at the time of writing has increased and the electrical power consumption of the magnetic storage apparatus is therefore increased.
Accordingly, it is an object of the present invention to provide a magnetic storage apparatus using a ferromagnetic tunnel junction device which can change the magnitude of writing magnetic force in order that the magnitude of the writing magnetic force is situated on the outside of the unstorable region of the ferromagnetic tunnel junction device even when the unstorable region is shifted.